The present invention relates to a nuclear magnetic resonance (NMR) machine phantom and a method of measuring the characteristics of a magnetic field using such a phantom. It finds its use more particularly in the medical field where such machines are used for helping diagnosis in man.
An NMR machine essentially includes means for producing a constant orientator magnetic field, intense and homogeneous in a zone of interest. A body to be examined is placed in this zone and the magnetic moments of the nuclei of the particles of this body tend to be aligned with this orientating field. Under these conditions, the body is subsequently subjected to radiofrequency excitation which causes the orientation of the magnetic moment to swing. When the excitation ceases, the magnetic moments are realigned with the constant field while restituting the energy received during the excitation. The restitution signal is picked up and is processed so as to reveal the structures of the internal parts of the body under examination. The response of the body is a volume response: all the particles subjected to examination emit their restitution signal at the same time. So as to differentiate the signals relative to different parts of the body the excitation is applied in the presence of a space coding. This coding consists in administering additional constant fields, different in different positions in space. In fact, the resonance phenomenon, at each position, depends on the amplitude of the constant field and on the characteristics of the particles examined. In the medical field the particles which it is desired to cause to resonate are the hydrogen atoms contained in water as well as in all organic substances, and whose concentration varies from one tissue to another.